1. Field of the Invention
This invention relates generally to a method for increasing regenerative braking in a hybrid electric vehicle.
2. Description of the Prior Art
The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In a hybrid electric powertrain an electric machine is coupled to the wheels through a torque converter and transmission gearing.
In the vehicle equipped with a modular hybrid transmission (MHT) and using a torque converter it is necessary to keep the torque converter bypass clutch locked during brake regeneration events. During such events an onboard electric machine is used as generator to recapture vehicle kinetic energy during braking.
Failure to keep the torque converter bypass clutch locked can result in the torque converter's impeller speed to rapidly decelerate to speeds where the transmission's mechanical pump can not operate efficiently. Such low speeds can result in loss of line pressure in the transmission's hydraulic system and loss of transmission clutch control.
Abandoning regenerative braking at higher speeds in anticipation of opening the torque converter bypass clutch results in kinetic energy that is not captured and thus in a reduced fuel economy benefit from regenerative braking.
In the previous solutions, regenerative braking is limited to the vehicle speeds and gears where the torque converter bypass clutch is sure to stay locked. During downshifts the amount of regenerative braking is limited to the level that would assure that torque converter bypass clutch slip is minimal. This also required a significant amount of blending, wherein torque smoothly transitions from reduced torque magnitudes to low negative regenerative torque magnitudes.